The present invention relates generally to a method and apparatus for collecting and transferring solar heat energy concentrated by an array of mirrors (heliostats).
Solar power tower systems use an array of several thousand heliostats to focus sunlight onto a central receiver mounted on top of a tower. When heat is collected at a temperature of >900° C. it can be used to power a thermo-chemical cycle that produces hydrogen at a much lower cost than conventional solar electrolysis. The leading candidate for the 900° C. solar receiver is known as the Solid Particle Receiver. In this receiver concept (illustrated in FIGS. 1-3) originally proposed by Sandia California in the mid-1980's, blackened alumina particles (the size of common beach sand) directly absorb the solar energy as they fall near the back wall within an open cavity (receiver).
A thermo-chemical (TC) process to produce hydrogen typically requires a heat input in excess of 900° C. This heat can be supplied by using either solar or nuclear energy. There has been significant work done to identify viable water splitting processes. Historically, processes receiving the most attention were those having a maximum temperature of around 900° C. that could be linked with a nuclear energy input. The Sulfur-Iodine and Hybrid Sulfur processes, developed by General Atomics and Westinghouse, respectively, are two examples. Processes requiring still higher temperatures are achievable only with a solar energy input. The Zinc-Oxide and Ferrite processes require heat input at a temperature of 2000° C. and 1600° C., respectively, and may be considered “solar-only”.
Integrating a TC process with a solar energy input requires a suitable solar interface. The prevailing wisdom with regard to the design of TC hydrogen production facilities is that bigger is better due to the economies of scale and operational considerations. Because of this, TC processes suitable for a central receiver platform tend to have more favorable economics. In addition, a central receiver facility has the potential to offer thermal storage and “around-the-clock” operation. The principal challenges involved with the central receiver solar interface for TC processes are 1) identifying a suitable heat transfer and storage media and 2) designing a receiver that is efficient at the required temperatures. The solid particle receiver addresses both of these challenges. “Around-the-clock” plant operation can be achieved by integrating 13 hours of solid-particle thermal storage into the design of the solar plant. Since solid-particles have high heat capacity and are relatively inexpensive, it is cost-effective to include storage into the design.
The Solid Particle Receiver (SPR) was initially studied in the early 1980's in an effort to create a direct absorption central receiver capable of interfacing with high temperature (>900° C.) electric power and chemical-production cycles. In its simplest configuration the SPR consists of a curtain of particles that are dropped through a beam of concentrated solar energy, within a cavity, and heated. The particles are typically dark in color and made of a ceramic material, such as sintered bauxite. The heated particles can then be stored and run through a heat exchanger to provide thermal energy input to a process. The dimensions of the receiver are quite large. As shown in FIG. 3, for a 350 MWth commercial receiver concept, the opening of the rectangular aperture can be as large as 15 meter wide by 11 meters tall; and the distance the sand falls can be as long as 12 meters. Particles entering the receiver are ‘cold’, about 600° C., and rise to 900-1000° C. when exiting to bottom.
Early work on the SPR was done primarily by Sandia National Labs (SNL) and focused on identifying an appropriate particle material with respect to optical properties and structural stability, evaluating the heat absorption characteristics of particle flows using a radiant heat source, and creating computational models to simulate receiver operation and aid in design efforts. This initial work at SNL was concluded in 1986, with the recommendation to proceed to on-sun testing on a central receiver platform. More recent SPR work included on-sun testing and optical characterization of a 2 MWt SPR prototype capable of achieving >300° C. at SNL.
In the absence of external winds, the particle curtain is stable when it falls from the top of the receiver cavity to a collection hopper at the bottom of the receiver cavity and is not expelled out the open aperture to the environment. However, calculations and experimental results indicate the curtain will become unstable when high-intensity external winds blow through the open cavity aperture, especially if the wind enters at oblique angles. Given these conditions, the particles would be blown out the open aperture. This would necessitate shutdown of the receiver. Since windy conditions are expected during many and perhaps most operating days, the receiver must be designed to operate in high winds. Not only can the wind cause a significant amount of net particle loss, but it can also move the curtain away from the receiver back wall and cause wall damage due to overheating.
Interestingly, we discovered that external wind blowing at normal incidence to the receiver (i.e., face-on) does not cause a problem with expelling particles; only when the wind is blowing obliquely.
An air curtain flowing directly across the aperture itself was tried, but didn't work to prevent particle loss.
Against this background, the present invention was developed.